Mixed solvent composition, insensitivity

Although Johnson and Furter (1,2), among others, observed a surprising insensitivity of k to mixed-solvent composition in many alcohol-water-inorganic salt systems, such does not appear to be the case with ammonium bromide-ethanol-water. A linear dependence of k with x was observed and is demonstrated in Figure 4. The slope of this dependence is 2.63 and the intercept with the y-axis occurs at approximately a value of unity. This extrapolated salt effect when x = 0, that is, with water as the single solvent, is consistent with Raoult s Law in that the vapor pressure of the aqueous salt solution depends directly on the salt concentration. However the same behavior has not been observed for the ammonium chloride-ethanol-water system (3) as seen in Table VIII its salt effect parameter shows essentially no dependence on the liquid composition. Therefore the two systems differ in this respect. 

The graphs shown in Fig. 2 are analogous to the master curves presented in Fig. 1 b, but to avoid overcrowding of data points, only the data points for one solid-to liquid ratio (2 % titania in this case) are explicitly shown in Fig. 2. Similar results were obtained for other solid-to-liquid ratios (1, 5, and 10 % Ti02), and for aqueous methanol (60-90 %). In the case of oxalic acid the lEP is rather insensitive to the solvent composition (20-94 % ethanol and 60-100 % methanol), and the lEP is at about 1.5 x 10" mol/m H2C2O4. The match between lEP obtained for different solvent compositions shown in Fig. 2 is a result of a fortuitous coincidence (interplay between and affinity of acid to the surface) rather than a general rule, and in principle the lEP is a function of solvent composition. Another set of zetametric curves in a series of mixed solvents of various compositions is presented in Fig. 3. To avoid overcrowding of data points. 

Gradients result from the mixing of two or more different solvents. The following restrictions apply to isocratic solvents selectivity, safety, sample compatibility, column compatibility, detector compatibility, viscosity, corrosivity, purity, cost. The same restrictions apply to gradient solvents, as well as, miscibility, mutual compatibility, and insensitivity of detector for variations in composition. 

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